1. Field of the Invention
This invention relates to linear filters. More particularly, it relates to architecture including a linear transversal equalizer.
2. Background of Related Art
Third Generation (3G) Universal Mobile Telecommunications System (UMTS) designs offer mobile operators significant capacity and broadband capabilities to support great numbers of voice and data customers—especially in urban centers. Making use of radio spectrum in bands identified by the International Telecommunications Union (ITU) and subsequently licensed to operators, 3G/UMTS employs a 5 MHz channel carrier width to deliver significantly higher data rates and increased capacity as compared with second generation networks. This 5 MHz channel carrier provides optimum use of radio resources, especially for operators who have been granted large, contiguous blocks of spectrum—typically ranging from 2×10 MHz up to 2×20 MHz—to reduce the cost of deploying 3G networks.
3G/UMTS has been specified as an integrated solution for mobile voice and data with wide area coverage. Universally standardized via the Third Generation Partnership Project (www.3gpp.org) and using globally harmonized spectrum in paired and unpaired bands, 3G/UMTS in its initial phase offers theoretical bit rates of up to 384 kbps in high mobility situations, rising as high as 2 Mbps in stationary/nomadic user environments. Symmetry between uplink and downlink data rates when using paired (FDD) spectrum also means that 3G/UMTS is ideally suited for applications such as real-time video telephony—in contrast with other technologies such as Asynchronous Digital Subscriber Line (ADSL) where there is a pronounced asymmetry between uplink and downlink throughput rates.
Specified and implemented as an end-to-end mobile system, 3G/UMTS also features the additional benefits of automatic international roaming plus integral security and billing functions, allowing operators to migrate from 2G to 3G while retaining many of their existing back-office systems. Offering increased capacity and speed at lower incremental cost as compared with second generation mobile systems, 3G/UMTS gives operators the flexibility to introduce new multimedia services to business users and consumers while providing an enhanced user experience. This in turn provides the opportunity for operators to build on the brand-based relationships they already enjoy with their customers—and drive new revenue opportunities by encouraging additional traffic, stimulating new usage patterns and strengthening customer loyalty.
Ongoing technical work within 3GPP will see further increases in throughput speeds of the WCDMA Radio Access Network (RAN). High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) technologies are already standardized and are undergoing network trials with operators in the Far East and North America. Promising theoretical downlink speeds as high as 14.4 Mbps (and respectively 5.8 Mbps uplink), these technologies will play an instrumental role in positioning 3G/UMTS as a key enabler for true ‘mobile broadband’. Offering data transmission speeds on the same order of magnitude as today's Ethernet-based networks that are a ubiquitous feature of the fixed-line environment, 3G/UMTS will offer enterprise customers and consumers all the benefits of broadband connectivity whilst on the move.
The linear transversal equalizer (LTE) has been one of the more encouraging technologies for receivers in high-speed data transmission, e.g. high speed downlink packet access (HSDPA) in communications systems conforming to standards promulgated by the Third Generation Partnership Project (3GPP). This is because of the simplicity of a linear transversal equalizer, and its ability to cancel inter-symbol interference (ISI).
A linear transversal equalizer is essentially a linear filter on a delay-line of received complex data. The linear transversal equalizer has multiple taps (i.e., samples), and each filter tap is multiplied by a complex weight.
FIG. 6 depicts a conventional linear transversal equalizer.
In particular, as shown in FIG. 6, a conventional linear transversal equalizer 600 includes multiple taps (i.e. samples) 602, 604, 606, each with a respective tap weight w0, w1, wN. Tap 604 is delayed from tap 602, and tap 606 is delayed from tap 604. The signal at each tap 602, 604, 606 is multiplied by its respective tap weight w0, w1, wN, as depicted by multipliers 610, 612, 614. The results of each of these multiplications, i.e., of each tap multiplied by its respective tap weight, in sum result in a total signal at output 650 having a given signal strength.
One of the difficulties faced by conventional linear transversal equalizers is that the transmitted signal spreads temporally due to multi-paths introduced by the channel as the receiver moves about.
Multi-paths are caused by reflections and other disturbances between the transmitter and the receiver. Differing paths of reflections cause the receiver to see multiple ‘sources’ of the same transmitted signal, the multiple ‘sources’ traveling over different paths. Since the paths are not all of the same length, the multiple reflections or ‘sources’ of a same transmitted signal may arrive at a receiver at slightly differing times.
As an example, FIG. 7 shows multi-paths in a conventional mobile communications system. As shown in FIG. 7, a transmitter 502 transmits a signal to a mobile receiver 504. To exemplify the multitude of paths that might be possible in any particular situation, two paths (1) and (2) are shown.
Path (1) first reflects off building 510, then off building 514, then off building 512, then off building 516, and finally reaches its destination receiver 504. A more direct path (2) reflects off only building 516 before reaching the receiver 504. Of course, it is also possible that a signal be directly received by the receiver 504 without any reflections. Needless to say, the different paths that a portion of the signal takes before being received by the receiver 504 each require a given amount of time to make the trip.
Multi-path movements are generally tracked by adjustments of tap weights on linear transversal filters (S. Qureshi, “Adaptive Equalization”, Processing of IEEE (1985)). As the delay of a particular path changes, the magnitude of the tap weight corresponding to the old delay is decreased, while the magnitude of the tap weight corresponding to the new delay is increased.
As the delay spread of the multi-paths moves beyond the span of the particular linear transversal filter, the signals taking paths that are positioned outside the filter span are not captured by the linear transversal equalizer. This loss of some portion of the original signal effectively reduces the total signal strength received by the receiver.
There is a need for reduced signal loss due to dropped multi-path signals, so that overall signal strength of a received signal through a linear transversal equalizer is increased.